Sargassum seaweed is an important source of food and shelter for many marine animals. But the algae can also be a nuisance when they wash up on coastlines: They can bury beaches, alter the coastal environment, and wreak havoc on tourism.
Scientists have now used satellite data to trace how a giant patch of Sargassum they’ve called the Great Atlantic Sargassum Belt grows and shrinks with changing environmental conditions. They suggest that higher levels of nutrient-rich discharge from the Amazon River and increased upwelling of cold, nutrient-laden water near West Africa are correlated with years of stronger algal blooms. These results are important for predicting Sargassum inundations, the researchers propose.
Spotting the Seaweed
Chuanmin Hu, an optical oceanographer at the University of South Florida in Tampa, and his team mined 19 years’ worth of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations to pinpoint floating Sargassum.
Researchers analyzed the visible and near-infrared channels of the imagery to locate the distinctive yellow-brown seaweed in over 53,000,000 square kilometers of the Atlantic Ocean and the Intra-Americas Sea (which includes the Gulf of Mexico and the Caribbean). To do so, Hu and his colleagues looked for the distinctive red edge reflectance of photosynthesizing plants, which is caused by chlorophyll’s enhanced reflectance in the infrared. (Astronomers hope to use the same technique one day to spot plants on distant planets orbiting other stars.)
Follow the Nutrients
Hu and his collaborators found negligible levels of Sargassum in the Caribbean Sea and the central Atlantic Ocean in the first decade of their data (2000–2010) but a pronounced increase in the prevalence of the algae in 2011. The uptick was likely caused by increased levels of nutrients such as nitrogen and phosphorus circulating high in the water column, Hu and his colleagues concluded. These nutrients predominantly came from two sources, the team hypothesized: fertilizer-polluted water discharged from the Amazon River and the upwelling of cold, nutrient-rich waters near West Africa.
Previously published research had demonstrated particularly strong nutrient discharge in 2009, but a mystery remained: Why did the Sargassum bloom occur in 2011 rather than 2010? Hu and his colleagues proposed that the higher-than-usual sea surface temperatures measured in 2010 were the culprit that caused the delay. Laboratory experiments have shown that warmer water suppresses Sargassum growth.
Sargassum blooms also occurred in 2015 and 2018, the MODIS data revealed. The 2018 event was the largest, Hu and his team found. In June of that year, the Great Atlantic Sargassum Belt stretched nearly 9,000 kilometers from West Africa to the Gulf of Mexico. Using estimates of Sargassum biomass gleaned from previous fieldwork, the researchers calculated that all of that algae probably weighed more than 20 million metric tons.
Some of that Sargassum washed up on shorelines. Thick layers of the seaweed choked beaches, creating a foul-smelling mess as it decomposed. The Caribbean nation of Barbados declared a national emergency and encouraged its residents to collect the seaweed to fertilize their gardens.
A New Normal
Although ocean currents play a role in moving seaweed, the real driver behind Sargassum’s spread in the tropical Atlantic Ocean is the availability of nutrients, Hu and his team concluded. Increased fertilizer use—Brazil consumed nearly 70% more fertilizer in 2018 than 2011—and widespread forest loss mean that nutrients build up in the Amazon and are more readily discharged.
All that adds up to more Sargassum in the ocean and potentially more common inundation events, the scientists noted. “Recurrent blooms in the tropical Atlantic and Caribbean Sea may become the new norm,” the researchers concluded in their study, which was published this month in Science.
“This study provides a clear picture of annual and seasonal Sargassum coverage,” said Amy Siuda, a biological oceanographer at Eckerd College in St. Petersburg, Fla., not involved in the research.
There’s a critical need to accurately forecast Sargassum inundations, and these results help reveal what drives these events, said Siuda. But there’s still more work to be done, she noted. “Like hurricanes or nor’easters, we will likely be able to predict the severity of the upcoming season, but we won’t be able to predict exactly where the inundations will come ashore.”
In the future, Hu and his team hope to do more fieldwork and gather more satellite data to better determine the extent and impact of Sargassum.
“What’s the impact on the environment?” asked Hu. “What are the effects of constant, extensive Sargassum inundations on human health?”
—Katherine Kornei, Freelance Science Journalist